In the manufacture of printed circuits, a circuit pattern is initially generated as a negative or positive image on a clear film. The image film serves as an artmaster from which numerous pieces of artwork, which embody the pattern, can be made. The artwork is then used in the actual manufacture of printed circuits.
During the manufacture of the artmaster, the circuit pattern must be located precisely and accurately dimensional on the film. This insures that the pattern of printed circuits, made by use of artwork generated from the artmaster, are precisely located on a supporting substrate.
To insure the accurate dimensioning and precise locating of the circuit pattern on the artmaster, datum marks are developed on the artmaster simultaneously with the circuit pattern and provide reference points relative to the circuit pattern. It is critically important that the datum marks be located at precise points on the artmaster and that the marks be spaced precise distances from each other. The datum marks then serve as locating reference points for the artwork in the ultimate manufacture of the printed circuits. The datum marks are also developed from the artwork onto the printed circuit substrate and provide the capability for ultimate positioning of the substrate to locate the circuit for additional processing (e.g., drilling holes, selective plating, etc.).
In one system of manufacturing the artmaster, the distance between and the location of the datum marks developed on the artmaster must be measured relative to the circuit pattern prior to the development of the artwork by use of the artmaster. In such a system, the artmaster is developed on the film and, while the film is in place, the distance between the datum marks is measured by use of a laser inferometer. The insure accuracy in the distance measurement, the inferometer must be precisely located over each of the datum marks during the measuring process.
In the manufacture of artwork, an unexposed film can be positioned on a movable table below a shutter/lens assembly. The artmaster is located between a light source and the assembly for selective exposure of the artmaster pattern onto the unexposed film. To insure accuracy of location of the pattern of the artwork, the artmaster must be precisely located relative to the table and the unexposed film prior to selective exposure. Still further, a single pattern developed on an artmaster can be developed at several locations on a single artwork. This provides artwork with a plurality of repetitive patterns which can be used to make the same plurality of printed circuits in one in-line procedure on a single substrate. Each of the plurality of circuit patterns will contain the datum marks developed from the artmaster. Again, the criticality of locating and aligning the datum marks is apparent.
One technique for centering an artwork reference target at a given location is disclosed in U.S. Pat. No. 4,012,148 which issued to W. F. Marantette and R. B. Marantette. As disclosed in the patent, an image of the artwork target is projected onto a rear projection screen. Four photocells are positioned adjustably along X-Y coordinates on the front face of the screen. Elongated photocells are initially located quadrilaterally about the intersection point on the orthogonal axes to sense ideally and equally light-projecting portions of the target when the target is in the given location. In use, as the target image is projected onto the screen, the photocells sense the position of the image, make comparisons of light sensed by the cells and facilitate control of servo motors to move the artwork until equal amounts of light are sensed by the photocells. At this time, the artwork is positioned in the given location.
U.S. Pat. No. 4,012,148 teaches the use of discrete photocells in the aligning system. Since each cell is separate and spaced from the other cells, each cell could be operating in a temperature environment different from that of the other cells. Also, since the cells are separate, they may not be matched for linearity which could result in variations in the comparative electrical outputs of the cells for equally sensed light intensity. Or, unequally sensed light could improperly provide indication of alignment of the target with the photocells. Further, the relatively large size required for individual cells indicates that the sensing characteristics of each cell may vary over the entire active area of the cell. With such variations in the sensing characteristics, it is difficult to obtain a uniform response when examining a target having fine-line definition.
Consequently, there is a need for a system with matched linearity and uniform response to accurately align a target of fine-line definition without concern for temperature variations.